Gas sterilization is widely used for medical devices that must be sterile at time of use, but cannot be subjected to high temperature sterilization. Examples of such medical devices include cardiac pacemakers and catheter-based monitoring devices, such as blood pressure probes. Typically, the medical device is sealed within a package that is permeable to gases but impermeable to bacteria. The package is then placed within a gas sterilization chamber, and a sterilizing gas, such as ethylene oxide, is first introduced into the gas-permeable package to achieve sterilization, and is then removed from the gas-permeable package, leaving the interior of the package sterile but non-toxic.
In a typical design, the medical device is placed within a thermoformed rigid plastic tray equipped with a flat sealing flange. A sheet of gas-permeable membrane, such as du Pont TYVEK.RTM. 1073-B (medical grade) brand membrane, which is available from E.I. du Pont de Nemours & Co., is then sealed to the sealing flange, typically by using an adhesive. TYVEK membrane is a porous material fabricated as a mat of polyethylene fibers. It is permeable to gases, but its pores are tortuous paths that exclude bacteria.
The integrity of the seal is critically important to maintaining sterility. Leaks can result from incorrect setting of parameters in the automated sealing process, or from physical defects such as burrs on the face of the sealing equipment.
In current known practice, human inspectors perform one hundred percent visual inspection of the finished packages to look for defective seals, and statistical samples undergo destructive testing using microbial challenge techniques. Each of these techniques has important drawbacks. Visual inspection may not detect every type of leak, and human visual inspection is subject to potential lapses of attention by human inspectors. In microbial challenge methods, the package first is exposed to an aerosol of bacteria. The outside of the package is then cleaned, the package is opened, and interior samples are tested by microbial culturing. The microbial challenge methods are less sensitive than visual inspection, and are subject to false positives due to contamination from the exterior of the package. Recent articles in Medical Device and Diagnostic Industry have discussed the limitations of microbial challenge methods. (J. Spitz, "Relevance of Microbial Challenge Testing to Package Sterility," (January 1994, p.142); J. Spitzley, "How Effective is Microbial Challenge Testing for Intact Sterile Packaging?" (August 1993, p.44).
In other applications, leak detection is often done by placing a tracer gas during packaging inside a package to be tested, and detecting low levels of the tracer gas outside the package. Detection of low levels of gas leakage is difficult, in which the package is specifically designed to be highly permeable to gases. More specifically, one portion of the package, the TYVEK membrane sheeting, is designed to be gas-permeable in the direction perpendicular to the plane of the membrane. It is of interest to detect gas-permeable flaws or channels in the seal between the TYVEK membrane and the sealing flange of the rigid plastic tray, and it is of interest to detect gas-permeable flaws in the rigid plastic tray, such as a pin hole.
Therefore, a need exists for a method and apparatus for non-destructive testing for a leak in a sealed package having an interior chamber formed of a gas-permeable portion and a non-gas-permeable portion.